The cellular slime mold, Dictyostelium discoideum, is a eukaryotic organism which serves as an important model for development and differentiation. It displays both a replicative vegetative stage and a separate non-replicative developmental phase. The major overall objective of this project is to characterize the processes of DNA repair in this organism following Co60-gamma irradiation during development. An understanding of the enzymatic repair of DNA, following its damage by radiation or mutagenic and carcinogenic chemicals is of importance in the assessment of the role of these agents in cell survival, mutagenesis, carcinogenesis, and aging in higher organisms. Our experiments will consist of: (1) a detailed study of our recently observed gamma-ray-induced unscheduled nuclear DNA synthesis during development, utilizing isopycnic CsC1 gradients to measure H3-thymidine uptake into the nuclear and mitochondrial DNA; (2) a determining of whether or not this unscheduled synthesis is repair synthesis, using H3- bromodeoxyuridine (BrdUrd) as a density label for the newly synthesized DNA; (3) measurement of the net repair during development after gamma irradiation via the photolysis by 313 nm light of the BrdUrd-containing repair regions, to yield single-strand breaks, as detected by alkaline sucrose gradients; (4) the utilization of the temperature-resistant diploid heterozygotes formed after the fusion of temperature-sensitive haploid strains of different radiation sensitivities, and subsequent haploidization accompanied by chromosome segregation, to perform a genetic analysis of the properties of our several radiosensitive strains; (5) Studies of the effect of gamma ray damage and its repair on the synthesis of new messenger RNA during development. These results will be correlated with the various steps of morphological development.